Understanding JPA, 5

Page 5 of 6

The power of inheritance

An entity can extend the following:

• Another entity -- either concrete or abstract.
• Another non-entity, supplying behavior or non-persistence state. The attributes you inherit from a non-entity are not persisted.
• Mapped superclasses, supplying common entity state. Tables in a database have similar fields, but tables are not related to each other.

Let's have a look into the various types of inheritance JPA offers. For this scenario, assume that there are two types of customer: a normal customer who buys products from a physical store and an online customer who buys products over the Internet.

Single-table inheritance

In single-table inheritance, all the entities in the hierarchy are stored in a single table. Single-table inheritance is the default strategy. Thus, you could omit the @Inheritance annotation in the example code in Listing 12 and get the same result.

In the example application, both ordinary and online customers are stored in the CUSTOMER table, as shown in Table 2.

Table 2. Single-table inheritance mapping strategy

ENTITY TABLE NAME

Customer	CUSTOMER
OnlineCustomer	CUSTOMER

The Customer entity has custId, firstName, lastName, custType, and address information, whereas the OnlineCustomer entity has only a website attribute and otherwise extends all features of Customer. This strategy should be reflected in the superclass, as in Listing 12.

Listing 12. A sample superclass in single-table inheritance

@Entity(name = "CUSTOMER") 
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="CUST_TYPE", discriminatorType=DiscriminatorType.STRING,length=10)
@DiscriminatorValue("RETAIL")
public class Customer implements Serializable{
    @Id 
    @Column(name = "CUST_ID", nullable = false)
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long custId;

    @Column(name = "FIRST_NAME", nullable = false,length = 50)
    private String firstName;

    @Column(name = "LAST_NAME", length = 50)
    private String lastName;

    @Embedded
    private Address address = new Address();

    @Column(name = "CUST_TYPE", length = 10)
    private String custType;
................
}

For the time being, ignore the DiscriminatorColumn and DiscriminatorValue annotations; you'll see how those work later. The OnlineCustomer entity will be a normal entity class that extends Customer class, as shown in Listing 13.

Listing 13. A sample subclass in single-table inheritance

@Entity(name = "ONLINECUSTOMER") //Name of the entity
@DiscriminatorValue("ONLINE")
public class OnlineCustomer extends Customer{
    @Column(name = "WEBSITE", length = 100)
    private String website;
    ............
}

Now you must create a Customer object and an OnlineCustomer object and persist them, as in Listing 14.

Listing 14. Persisting objects in single-table inheritance

......................
userTransaction.begin();
//inserting Customer
Customer customer = new Customer();
customer.setFirstName("Charles");
customer.setLastName("Dickens");
customer.setCustType("RETAIL");
customer.getAddress().setStreet("10 Downing Street");
customer.getAddress().setAppt("1");
customer.getAddress().setCity("NewYork");
customer.getAddress().setZipCode("12345");
em.persist(customer);
//Inserting Online customer
OnlineCustomer onlineCust = new OnlineCustomer();
onlineCust.setFirstName("Henry");
onlineCust.setLastName("Ho");
onlineCust.setCustType("ONLINE");
onlineCust.getAddress().setStreet("1 Mission Street");
onlineCust.getAddress().setAppt("111");
onlineCust.getAddress().setCity("NewYork");
onlineCust.getAddress().setZipCode("23456");
onlineCust.setWebsite("www.amazon.com");
em.persist(onlineCust);
userTransaction.commit();
......................

If you have a look into the CUSTOMER table now, you will find two records. The query in Listing 15 should return you the list of online customers from the data store.

Listing 15. Fetching only the subclass in single-table inheritance

..............
Query query = em.createQuery("SELECT customer FROM ONLINECUSTOMER customer");
List<OnlineCustomer> list= query.getResultList();
.................

If the CUSTOMER table stores both the Customer and the OnlineCustomer data, how will JPA distinguish one from the other? How will it fetch only the online customers? In fact, JPA cannot do this unless you provide it with a hint. That's the significance of the @DiscriminatorColumn. It tells the CUSTOMER table which column distinguishes a CUSTOMER from an ONLINE CUSTOMER. @DiscriminatorValue indicates what value identifies a CUSTOMER and an ONLINE CUSTOMER. The @DiscriminatorValue annotation needs to be provided in the superclass as well as in all the subclasses.

When you want to fetch online customers, JPA silently queries the data store as in Listing 16.

Listing 16. Distinguishing objects stored in a single table

SELECT t0.CUST_ID, t0.CUST_TYPE, t0.LAST_UPDATED_TIME, t0.APPT, t0.city, t0.street, t0.ZIP_CODE, t0.FIRST_NAME, t0.LAST_NAME, t0.WEBSITE FROM CUSTOMER t0 
WHERE t0.CUST_TYPE = 'ONLINE'